Temperature compensation circuit for a multi-frequency receiver

ABSTRACT

A frequency detection circuit is driven by an input signal limiter circuit, which is temperature compensated by a zener diode having a negative temperature coefficient, and includes a threshold detection circuit, which is temperature compensated by a zener diode having a positive temperature coefficient.

United States Patent 11 1 Tanaka et al.

1 1 Jan. 14, 1975 TEMPERATURE COMPENSATION CIRCUIT FOR A MULTI-FREQUENCYRECEIVER [75] Inventors: Yuji Tanaka; Matsuo Takaoka, both of Kawasaki;Kazuhiro Gosho, Yokohama, all of Japan [73] Assignees: Nippon Tsu ShinKogyo K.K.,

Kanagawa-ken, Japan; TIE/Communications Inc., Stamford, Conn.

[22] Filed: Apr. 5, 1973 [21] Appl. No.: 348,100

[30] Foreign Application Priority Data Primary ExaminerAlfred L. BrodyAttorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin [57]ABSTRACT A frequency detection circuit is driven by an input sig- Jan.13,1973 Japan 48/12512 nal limiter circuit, which is temperaturecompensated by a zener diode having a negative temperature coeffi- 52 11 U S C 329/136 307/310 4 clent, and lncludes a threshold detectionclrcuit, [51] Int Cl H03c 3/04 which is temperature compensated by azener diode 53 Field 01 Search 329/110, 143, 136, 131; having a lmsmvetemperature coefficien 307/310 3 Claims, 14 Drawing Figures l o--LIMITER iNVERTER o 4 Vs i 202 Z0 5 PATEHTED JAN 1 4|975 SHEET 1 UF 3INVERTER LIMITER PATENTED JAN 1 4|975 SHEET 2 BF 3 PATENTEB JAN 1 M975SHEHSUFS INQ PN Q0 TEMPERATURE COMPENSATION CIRCUIT FOR AMULTI-FREQUENCY RECEIVER BACKGROUND OF THE INVENTION The inventionrelates to frequency detection circuits and in particular to frequencydetection circuits for use in DTMF receivers for converting dual tonemultifrequency (DTMF) dial signals into d-c contact closures.

Specifically, the invention relates to temperature compensation of suchdetection circuits to minimize recognition bandwidth variations withrespect to temperature. In effecting such compensation conventionally,the capacitors and coils incorporated in the tuning circuit are selectedto provide mutually complementary temperature coefficients. In addition,fixed voltages are utilized for establishing threshold levels in thecircuit to obtain further improvement.

However, as a practical matter, selection of temperature coefficients ofcoils and capacitors alone does not eliminate variations in therecognition bandwidth because the temperature coefficients of the coilscan not be exactly complementary to those of the capacitors. Moreover,zener diodes are utilized to obtain fixed the threshold levels. Sincethe diodes themselves exhibits a voltage breakdown characteristic whichvaries with respect to temperature, undesirable variations in thefrequency recognition bandwidth result.

SUMMARY OF THE INVENTION It is, therefore, an object of the invention toprovide a simple and inexpensive temperature compensated frequencydetection circuit.

In accordance with one aspect of the invention, a frequency detectioncircuit comprises in combination means, including a zener diode, forregulating the output level of the input signal limiter circuit meansdriving the detection circuits, the zener diode having a negativetemperature coefficient to compensate for the positive temperaturecoefficient of the limiter circuit.

In accordance with a second aspect of the invention, a frequencydetecting circuit comprises means including a negative temperaturecoefficient zener diode for compensating the positive coefficients ofelements of the detector circuits.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an electrical schematic andblock diagram of a temperature compensated frequency detection circuitconstructed in accordance with the invention;

FIG. 2 is a graph illustrating the detection circuit recognition banddisplacement shown by the output level as a function of frequency atthree temperatures;

FIG. 3 is a graph illustrating bandwidth variation resulting fromchanges in Q factor of the detection circuits tuning network, shown bythe output level as a function of frequency at three temperatures;

FIG. 4 is a graph illustrating the output voltage as a function oftemperature for the output of an uncompensated limiter circuit.

FIG. 5 is a graph illustrating the forward conducting voltage as afunction of temperature for the diode D1 shown in FIG. 1;

FIG. 6 is a graph illustrating the base forward conducting voltage as afunction of temperature for the transistor TRl shown in FIG. 1;

FIG. 7 is a graph illustrating the zener voltage as a function oftemperature for the zener diode ZDI shown in FIG. 1;

FIG. 8 is a graph illustrating the zener voltage as a function oftemperature for the zener diode ZD2 shown in FIG. 1;

FIG. 9 is a graph illustrating bandwidth variation of the detectioncircuit caused by threshold drift due to the temperature characteristicsof the uncompensated limiter circuit;

FIG. 10 is a graph illustrating bandwidth variation of the detectioncircuit caused by threshold drift due to the temperature characteristicsof the diode D1;

FIG. 11 is a graph illustrating the bandwidth variation of the detectioncircuit caused by threshold drift due to the temperature characteristicsof transistor TRl;

FIG. 12 is a graph illustrating the bandwidth variation of the detectioncircuit caused by the compensating temperature characteristics of zenerdiode ZDl;

FIG. 13 is a graph illustrating the bandwidth variation of the detectioncircuit output caused by the compensating temperature characteristics ofzener diode ZD2; and

FIG. 14 is a graph showing the ideal remaining recognition banddisplacement variation without bandwidth variation of the detectioncircuit compensated in ac-' cordance with this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The temperaturecompensated frequency detection circuit shown in FIG. 1 has generalapplication in detector circuits in telephone systems employing voicefrequency signalling techniques and has particular utility inmulti-frequency address signalling system receivers well known to thetelecommunication art, such as subscriber dual-tone multi-frequencypulsing.

Referring to FIG. 1, a sinusoidal input signal, whose frequency isnumerically representative, is applied to the input terminals 1, 2 of atemperature compensated frequency detector constructed in accordancewith the invention. If the frequency of the input signal lies within therecognition band of the tuned series resonant circuit Ll, C2 thedetection circuit develops a d-c output signal across output terminals4, 5.

In particular, the input signal applied to terminal I, 2 is fed to aconventional limiter circuit 10, which converts the input signal into anoutput signal of rectangular waveform andfixed voltage level, andcouples the output signal to a tuned circuit, which may comprise theseries tuned circuit resistor R2, capacitor C2 and inductor L1.

The characteristics of capacitor C2 and inductor L] are selected toprovide series resonance at the frequency to be detected. Resistor R2provides a means for controlling the Q factor of this circuit. Moreover,the temperature coefficients are selected to provide the closestpossible complementary temperature match. Typically, capacitor C2 is ofpolystyrole dielectric, and inductor Ll comprises a coil winding on aferrite core. If the coil temperature coefficient is somewhat greaterthan that of the capacitor, the tuning center frequency will varyinversely with respect to temperature, as shown in FIG. 2. The Q willalso vary inversely with respect to temperature, as shown in FIG. 3.

As shown in FIGS. 4, 9, an uncompensated limiter circuit output voltage10 has a positive temperature coefficient (FIG. 4); the bandwidth of thefrequency detection circuit would increase with temperature as a resultof this characteristic (FIG. 9), that is, the detection circuit wouldbecome more sensitive as temperature increases. However, the limitercircuit output level is compensated by zener diode 2D]. The cathode ofdiode ZDI is coupled through resistor R1 to a suitable source of d-cpower applied between terminals 3, of the detection circuit. CapacitorC1 provides an a-c bypass path for diode ZDl.

In accordance with one aspect of the invention, the zener diode ZDlselected for incorporation in the circuit is of the low voltage kind,and has a negative temperature coefficient, FIG. 7, causing acorresponding limiter output voltage correction, FIG. 12 to complementthe positive temperature coefficient of the limiter circuit itself (FIG.9).

When the limiter circuit 10 output signal lies within the recognitionband of series resonant circuit L1, C2, and when the peak value of thepositive half-cycle of the voltage across coil Ll exceeds the sum ofthree voltages, which are; the forward conducting voltage (V of diodeD1, the forward conducting base-emitter voltage (V,,,;) of transistorTRl, and the zener voltage (V of diode ZD2, transistor TRl is switchedon. During its period of conduction, the charge previously stored incapacitor C3, rapidly discharges through the collector-emitter circuit.

During the negative half-cycle of the voltage across inductor L1,capacitor C3 is recharged via the path to the dc power supply comprisingresistor R4. As a result of this on-off-on operation of transistor TRl,a low level d-c signal is obtained at the collector of transistor TRI.This signal is corrected by the conventional inverter circuit 11, and issupplied thereby to the detection circuit output terminals 4, 5.

As stated above, the conduction threshold level of transistor TRl is setby the zener voltage (V of diode ZD2. The cathode thereof is coupledthrough a suitable resistor R5 to the d-c power source at terminals 3,5. Capacitor C4 provides an a-c bypass path for diode ZD2.

As illustrated in the drawings, the temperature coefficients ofrectifier diode D1 (FIG. 5) and the baseemitter junction or transistorTRl (FIG. 6) are negative. Consequently, the bandwidth of the detectioncircuit, by virtue of these negative coefficients, increases withtemperature increases, that is, the detection circuit sensitivityincreases with temperature increases as can be seen from FIGS. 10 and11.

In accordance with another aspect of the invention, the zener diode ZD2,selected for incorporation in the detection circuit, is of the highbreakdown voltage kind, and has a positive temperature coefficient, FIG.8, to compensate for the negative temperature coefficients of diode D1and transistor TRl. FIG. 13 shows the compensation introduced by thecharacteristics of ZD2, complementing FIGS. 10 and 11.

Accordingly, the only characteristics of the detection circuit which arenot independently compensated are those of the tuned circuit elements,inductor L1 and eapacitor C2. As noted above, these can be appropriatelyselected with respect to each other to obtain almost complementarytemperature characteristics to achieve a detection circuit centerfrequency drift characteristic (FIG. 2) as close to ideal as possible.Moreover, temperature compensation of the threshold level in accordancewith this invention will limit the recognition bandwidth shift to thatshown by FIG. 14.

It will be appreciated by those skilled in the art that the temperaturecompensated frequency detection circuit disclosed herein can be used todetect periodic input signals of non-sinusoidal waveform, and that withappropriate modification, well within the skill of those in the art,shunt resonant networks, or other equivalent tuning circuits, and P-N-Ptransistors or equivalent switching devices, can be used in place of thespecific tuning circuit and N-PN transistor described above.

Moreover, while specific embodiments of the invention have beendisclosed, variations in procedural and structural detail within thescope of the appended claims are possible, and are contemplated. Thereis, therefore, no intention of limitation to the abstract, or the exactdisclosure herein presented.

What is claimed is:

1. A frequency detection circuit comprising: an input amplitude limitercircuit means having a constant output signal level over a predeterminedrange of input signal levels, and having a positive output signal leveltemperature coefficient; at least one tuned bandpass circuit means forcoupling said output signal to the next mentioned means; and at leastone threshold detection means having a negative threshold leveltemperature coefficient for deriving a DC output signal in response tothe output of said tuned circuit lying within the bandwidth thereof,wherein the improvement comprises, in combination, means, coupled tosaid limiter means and including a first zener diode means having anegative temperature coefficient, for setting the output level of saidlimiter circuit means independently of its temperature by compensatingfor the positive temperature coefficient of said limiter circuit means;and means, coupled to said threshold detection means and including asecond zener diode means having a positive temperature coefficient, forsetting the conduction level of said threshold detection means, and forcompensating for the negative temperature coefficient of said thresholddetection means.

2. A frequency detection circuit comprising in combination:

means, having an output port and being responsive to input signalssupplied to said detection circuit, for deriving a first signal having arectangular waveform and the same period as said input signal, saidmeans having a positive output voltagetemperature coefficient;

means, coupled to said deriving means and including a first zener diodemeans having a negative voltage temperature coefficient, for setting theoutput voltage of said deriving means independently (a) of thetemperature of said deriving means and (b) of the level of the inputsignals supplied thereto:

means for detecting an input signal lying within a preselected band offrequencies, coupled to said output port of said deriving means andincluding in serial connection, a network means tuned to reject signalsof frequency outside said band and arranged to pass signals of frequencywithin said band; means having a negative conductancevoltage-temperaturecoefficient for rectifying the positive half-cycles of the voltagedeveloped across at least one energy storage element in said tunednetwork means; and means, responsive to the output of said rectifyingmeans for developing a D-C cient of said rectifying means, whereby theoutput sensitivity of the frequency detection circuit with respect totemperature variations, is primarily substantially dependent upon thefrequency band drift with respect to temperature of said tuned networkmeans and independent of the temperature dependent frequency variationsof said deriving means and said detecting means.

3. The frequency detection circuit according to claim 10 2 wherein saidtuned network means comprises a series resonant circuit means, andfurther wherein said one energy storage element comprises an inductancemeans.

1. A frequency detection circuit comprising: an input amplitude limitercircuit means havinG a constant output signal level over a predeterminedrange of input signal levels, and having a positive output signal leveltemperature coefficient; at least one tuned bandpass circuit means forcoupling said output signal to the next mentioned means; and at leastone threshold detection means having a negative threshold leveltemperature coefficient for deriving a D-C output signal in response tothe output of said tuned circuit lying within the bandwidth thereof,wherein the improvement comprises, in combination, means, coupled tosaid limiter means and including a first zener diode means having anegative temperature coefficient, for setting the output level of saidlimiter circuit means independently of its temperature by compensatingfor the positive temperature coefficient of said limiter circuit means;and means, coupled to said threshold detection means and including asecond zener diode means having a positive temperature coefficient, forsetting the conduction level of said threshold detection means, and forcompensating for the negative temperature coefficient of said thresholddetection means.
 2. A frequency detection circuit comprising incombination: means, having an output port and being responsive to inputsignals supplied to said detection circuit, for deriving a first signalhaving a rectangular waveform and the same period as said input signal,said means having a positive output voltage-temperature coefficient;means, coupled to said deriving means and including a first zener diodemeans having a negative voltage temperature coefficient, for setting theoutput voltage of said deriving means independently (a) of thetemperature of said deriving means and (b) of the level of the inputsignals supplied thereto: means for detecting an input signal lyingwithin a preselected band of frequencies, coupled to said output port ofsaid deriving means and including in serial connection, a network meanstuned to reject signals of frequency outside said band and arranged topass signals of frequency within said band; means having a negativeconductance-voltage-temperature coefficient for rectifying the positivehalf-cycles of the voltage developed across at least one energy storageelement in said tuned network means; and means, responsive to the outputof said rectifying means for developing a D-C output signalrepresentative of a detected input signal lying within said band; andmeans, coupled to said detecting means and including a second zenerdiode means having a positive voltage temperature coefficient, forsetting the threshold level of conduction of said rectifying means; theenumerated means being so proportioned and the combination being soconstructed and arranged that the said temperature coefficient of saidfirst zener diode means complements the said temperature coefficient ofsaid deriving means, and the said temperature coefficient of said secondzener diode means complements the said temperature coefficient of saidrectifying means, whereby the output sensitivity of the frequencydetection circuit with respect to temperature variations, is primarilysubstantially dependent upon the frequency band drift with respect totemperature of said tuned network means and independent of thetemperature dependent frequency variations of said deriving means andsaid detecting means.
 3. The frequency detection circuit according toclaim 2 wherein said tuned network means comprises a series resonantcircuit means, and further wherein said one energy storage elementcomprises an inductance means.